This invention relates generally to a method for enhancing the adhesion of a diamond-like carbon (DLC) film to a substrate and for producing a strongly adhered DLC film on a metallic substrate or a metallic substrate having an oxide film on the surface and, more particularly, to cases where the substrate intended to receive the DLC film comprises a metal or metal alloy that does not readily form carbides or a metal or metal alloy that may form carbides, but has an oxide coating that does not readily form bonds with carbon. The invention causes strongly adhered DLC films on such metallic substrates or oxide films.
In order to improve the wear resistance or other properties of the surfaces of various substrates, it is desirable to overlay the substrate with a film of a harder material such as diamond-like carbon (DLC). Since the one of the goals in applying a hard DLC film on a substrate is to improve durability and lifetime, it is important that the hard DLC film be strongly adherent to the substrate so that it does not prematurely separate from it.
Of course, it is recognized that these concerns occur in many fields of application where it is desirable to improve adhesion of DLC films to metallic substrates or metallic films that have been deposited on other substrates, and especially so in cases where the substrate metal does not easily adhere to deposited DLC because it does not readily form strong carbide bonds at low temperatures, such as chromium, cobalt, nickel, copper, and alloys thereof or predominately thereof. An example is a magnetic data storage disk substrate having a chromium or copper film surface on which it is desired to deposit a durable DLC protective film coating. Another case is titanium or an alloy consisting primarily of titanium, but having an oxidized surface layer, which may be a native oxide layer. Although titanium may form carbide bonds, the oxide surface layer does not readily form strong carbide bonds at low temperatures and thus strongly adhered DLC films are not readily formed.
One of the problems encountered is that the DLC, when applied by conventional methods like chemical vapor deposition (CVD), physical vapor deposition (PVD), pulsed laser deposition (PLD), conventional ion beam assisted deposition (IBAD), or gas cluster ion beam assisted deposition (GCIBAD), does not produce DLC films that are sufficiently strongly adherent to the substrate metals and which therefore do not have long life and high wear resistance. Conventional methods investigated for improving the poor adhesion of a film include ion beam interface stitching, substrate pre-sputtering, post deposition interface ion implantation, and ion beam assisted deposition. These methods are all described by J. Baglin, in xe2x80x9cInterface structure and thin film adhesionxe2x80x9d, in Handbook of Ion Beam Processing Technology -Principles, Deposition, Film Modification and Synthesis, edited by J. Cuomo et. al., Noyes Publications, Park Ridge, N. J. (1989).
Furthermore, interface stitching and post-deposition interface ion implantation both require the ion beams employed be of sufficient energy to completely penetrate the deposited DLC film. This imposes thickness limitations on the DLC films due to the practical unavailability of ion beams of arbitrarily high energies. Other, more complex, treatments involve forming one or ore intermediate layers between the substrate metal and the DLC film of, for example, silicon (taught in U.S. Pat. No. 5,593,719, G. Dearnaly et. al., xe2x80x9cTreatments to reduce frictional wear between components made of ultra-high molecular weight polyethylene and metal alloysxe2x80x9d, 1997) or germanium (taught in U.S. Pat. No. 5,780,119, G. Dearnaly et. al., xe2x80x9cTreatments to reduce frictional wear on metal alloy componentsxe2x80x9d 1998) or a silicon compound or the like (as taught in U.S. Pat. No. 5,605,714, G. Dearnaly et. al., xe2x80x9cTreatments to reduce thrombogeneticity in heart valves made from titanium and its alloysxe2x80x9d 1997). In the prior art, when an oxide film covers a metal or metal alloy that otherwise might form carbide bonds, steps were required to remove the oxide film prior to DLC deposition and such steps often require the application of high temperatures (as also taught in U.S. Pat. No. 5,605,714, G. Dearnaly et. al., xe2x80x9cTreatments to reduce thrombogeneticity in heart valves made from titanium and its alloysxe2x80x9d 1997).
Such treatments, however, are complex and time consuming and thus, costly. Also, processes that employ additional materials such as silicon, germanium, or the like introduce foreign materials (Si, Ge, etc) which may detract from the suitability of the resulting structure for some applications. Furthermore, some of the existing processes require heating of the substrates and films during portions of the process, which allows the possibility of thermal degradation of materials or that differing thermal coefficients of expansion between substrate and film result in the introduction of stresses into the film when the materials are returned to room temperature.
In the example of a magnetic storage disk, there may be multiple stacked films of differing materials on the disk""s base substrate. In such case, it is desirable to be able to deposit a DLC protective film without subjecting the disk to large temperature excursions that may produce undesirable results due to the different temperature coefficients of expansion of the various layers.
It is therefore an object of this invention to provide a method and system for producing a strongly adhered DLC coating for metals and metal alloys.
It is a further object of this invention to provide a method for causing improved adhesion of a DLC film to a substrate that does not readily form carbide bonds at low temperatures.
It is an additional object of this invention to provide DLC coating and improved DLC film adhesion by a process that is less costly than previous methods.
It is a further object of this invention to provide DLC coating and improved DLC film adhesion by a process that does not require high processing temperatures.
It is an additional object of this invention to provide a DLC film coating on metal or metal oxide substrates without introducing foreign materials.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.
A substrate, being comprised of a metal or metal alloy that does not readily form carbides (for example chromium, cobalt, nickel, copper, and alloys thereof or predominately thereof) or a metal or metal alloy that may form carbides but is coated with an oxide coating that does not readily form bonds with carbon (for example titanium or an alloy consisting primarily of titanium, but having an oxidized surface layer) receives an ordinary solvent or chemical or other cleaning to assure surface cleanliness. It is then subjected to a pre-deposition adhesion enhancement process that includes the procedure of ion implantation of the surface of the metal substrate to which the DLC film will be deposited with carbon ions at a dose and energy sufficient to establish a volume concentration of carbon atoms at and near the surface of the substrate of not less than 5xc3x971018 atoms/cm3, and preferably of more than 4xc3x971019 atoms/cm3. Although the implantation energy is not critical, a preferred method is to implant the carbon ions at an energy of from 25 keV to 100 keV. The implantation may be done at room temperature or somewhat above room temperature as may result naturally from slight heating resulting from the implantation, with or without active cooling of the substrate. The carbon implantation is preferably done on an ion implanter that has mass analysis to select 12C and reject other ion speciesxe2x80x94this assures that only pure carbon is introduced to the substrate being processed, thus avoiding the introduction of materials other than the carbon which forms the DLC that will be subsequently applied.
Although implantation of carbon is part of the present invention, the use of the 12C isotope of carbon is not essential. 13C or a mixture of 12C and 13C or a natural abundance mixture of carbon isotopes is acceptable. 12C is preferred, however, if a single isotope is used, because of its high natural abundance. Many carbon-bearing compounds can serve as the source of carbon ions and the selection of the source material is somewhat dependent on the ion source employed, but in the case of the ion implantation tool I have used with this invention, it has been convenient to use a gaseous source. CO, CO2, and methane are examples of suitable gaseous source materials. CO is preferred because of its high atomic percentage of carbon, which results in an improved carbon ion yield in many ion sources.
It is acceptable for the implantation to be performed with the substrate at temperatures between 0 and 300xc2x0 C. If it is desired or advantageous that the substrate be maintained at temperatures below a limiting temperature during processing, this may be accomplished by traditional methods such as heat sinking the substrate to a sufficient thermal mass to limit heating, by active cooling of the substrate by conduction through a cooled holder, or by limiting the implantion beam current to a maximum that will assure the temperature of the workpiece (substrate) does not exceed the limit temperature, for example 50xc2x0 C. The implantation energy is in the range of from 200 eV to 200 keV, with a preferred energy range being 25 to 100 keV. The implantation process imbeds carbon atoms into the subsurface region of the substrate so that carbon-carbon bonds may form between the substrate and the subsequently deposited DLC film, thus improving the adhesion over that which occurs without the preprocessing. Also, the substrate surface is roughened by the radiation damage induced by the implantation, which roughness additionally improves the adhesion of the subsequently deposited DLC film.